From The Cook's Bible: How to Thicken a Sauce

The simple science of how to thicken a sauce without adding calories or sacrificing on flavor.

Step 1: Forget most of what you know about classic French cooking.

If you have ever taken a course in classic French cooking, forget almost everything they told you. The French, with great intestinal fortitude, thickened sauces with butter, flour, egg yolks, and cream, to name a few of the culprits. They whisk together butter and flour (a roux) and then add in milk (a béchamel) or stock (a velouté), which thickens almost immediately. They use a paste, or little balls, of equal parts of flour and butter (beurre manié). These were added to sauces or stews as a thickener near the end of cooking. They also used egg yolks whisked into a sauce and reduced cream (cream that has been boiled down to thicken it).

Butter can also be used as a thickener. When it is whisked into a hot liquid, the milk solids and proteins in butter form an emulsion that suspends the particles of fat in the liquid, creating a thicker, shinier sauce. The problem with this last method, however, is that you need lots of butter to make this work (1/2 cup of butter is needed per 1 cup of liquid--1/2 cup of butter is an entire stick!). Nobody in their right mind is going to thicken with butter these days. Other thickeners include blood, foie gras (goose liver), yogurt, fresh cheese, and bread. Prior to the 16th century, coarse bread and ground almonds were the thickeners of choice.

So much for haute cuisine. The quickest and easiest thickener for a home cook is cornstarch. Whisk it with equal parts of water and then add it to a stock or a gravy and you have an instant sauce. This is how most people thicken gravy for Thanksgiving. But there are three good alternatives. Since most thickeners are starch, the two most obvious thickeners are potatoes and rice. Potatoes are particularly well suited to soups and stews (cook extra potatoes with the dish and then purée some of the liquid with the excess potatoes). I often keep extra cooked rice in the refrigerator as a thickener as well. I use it with stock and puréed vegetables for a quick and light sauce. I also use stale white bread, which is the traditional thickener for a rouille, a sauce made with roasted red peppers, lots of garlic, and hot peppers.

The Science of Cooking: Why do flour and cornstarch thicken sauces?

You've probably noticed that when stirred into water at room temperature, neither flour nor cornstarch thicken. They just turn into a loose paste. What is missing is the application of heat, which encourages the bonding of starch and water molecules. (Both flour and cornstarch are mostly starch, although flour contains many other ingredients such as proteins.) The starch granules then start to enlarge (think of blowing up a beach ball), trapping water as they grow. Finally, at temperatures over 150 degrees and up to a point just below boiling, the rigid structure of the granules breaks up, creating a spidery web of bonded starch and water molecules. This mesh prevents the free movement of water molecules and results in a thick sauce. You probably have also noticed that at this point, the sauce starts to become clearer. That is because the starch molecules are no longer packed tightly together--they are in a looser meshwork after heating--and therefore light is less likely to be deflected. At temperatures above 200 degrees, however, the large starch granules start to shrink in size, leaking starch molecules into the sauce. As these swollen granules deflate, the sauce becomes thinner.

Although flour is the traditional thickening agent in French cooking, cornstarch is a more powerful thickener because it is a purer form of starch. It will also create a clearer, shinier sauce. The French clarified their sauces through hours of slow cooking, skimming off the protein as the sauce simmered, and turning them into perfectly clear, shiny liquids, which keep their shine even with the addition of flour. Given the realities of home cooking, this makes no sense and, therefore, cornstarch is preferable.

The White Sauce

The most basic sauce is based on a roux, which is equal volumes of butter and flour whisked together in a saucepan over heat in order to ameliorate the flavor of the flour and to remove lumps.

To make a sauce or a base for a soufflé, for example, hot milk, cream, or stock is added and, in just a few minutes, the liquid thickens up nicely. The thickness of the sauce is easily varied by changing the proportion of roux to liquid. For a thinner sauce, a French chef uses 1 tablespoon each of butter and flour to 1 cup of liquid; for a medium sauce, 2 tablespoons each; for a thick sauce, 3 tablespoons. These two basic sauces, a béchamel and a velouté, are the basis for classic French white sauces.

By adding additional ingredients, such as cream, cheese-flavored butters, herbs, tomatoes, egg yolks, curry, white wine, lemon, onions, peppers, etc., or combinations thereof, chefs made more than 20 different sauces, everything from an aurore sauce (with tomato purée) to a Mornay sauce (with cheese). Although these two mother sauces are important as ingredients in recipes, I find them relatively useless for sauces. They are too heavy, too thick, and too high in fat. You do need, however, a béchamel for a soufflé, and a béchamel or velouté is also used in a classic lasagna, along with a basic tomato sauce.

Recipe for Mushroom Purée - You can add a mushroom purée to almost any sauce.